On Thu, 15 May 2003, Mark Radabaugh wrote:
I'm having a hard time finding best practices for filtering outbound bgp announcements when providing transit to bgp-speaking customers. While we currently multi-home to several providers it appears we will soon need to provide transit for customers with their own AS's.
I find lots of references (and understand) the basic
ip as-path access-list 3 permit ^$
and it would seem that should we wish to provide transit for a bgp customer AS12345 we would use:
ip as-path access-list 3 permit ^12345$
but I think this breaks if AS12345 prepends their advertisement.
yes it will
Next up is:
ip as-path access-list 3 permit ^12345_[0-9]$*
Which seems correct to me. Is this still best practice (or even correct)?
no, perhaps you mean ..[0-9]*$ but that still wont allow multiple prepends as it wont match the space only the numbers try ip as-path access-list 3 permit ^(_12345_)+$ which will allow one or more of their as's ASN is pretty crude tho, consider using a prefix list to filter each prefix and include length Steve
Mark Radabaugh Amplex (419) 720-3635